NASA

Technical

Memorandum

NASA TM- 100394

THE CORROSION PROTECTION OF 6061-T6 ALUMINUM BY

A POLYURETHANE-SEALED ANODIZED COAT

By M.D. Danford

Materials and Processes Laboratory

Science and Engineering Directorate

April 1990

('Ws:i';A- i '_- " C._3_, '::-) T;!:

'_ f'_,l_ Y_J_ IHA'<qL-.,"ALL:"

_3, _ ¢.SCL lie

National Aeronautics and

Sl:)ace Administration

George C. Marshall Space Flight Center

MSFC- Form 3190 (Rev. May 1983)

Report Documentation Page

: ..... I-,,,, I. ,I ,r,

1. Report No I 2. Government

NASA TM-100394 L

4. Title and Subtitle

Accession No.

The Corrosion Protection of 6061-T6 Aluminum By

A Polyurethane-Sealed Anodized Coat

-_).Performing()rganizationNameandAddress

George C. Marshall Space Flight Center

Marshall Space Flight Center, Alabama 35812

i2. SponsoringAgencyName-an-d-Add-iessNational Aeronautics and Space Administration

Washington, D.C. 20546

3. Recipient's Catalog No.

5. Report Date

April 1990

6. Performing Organization Code

8. Performing Organization Report No.

10. Work Unit No.

11. Contract or Grant No.

13. Type of Report and Period Covered

Technical Memorandum

14. Sponsoring Agency Code

Prepared by Materials and Processes Laboratory, Science and Engineering Directorate.

16. Abstract

The corrosion protection of 6061-T6 anodized aluminum afforded by a newly patented

polyurethane seal l_s,been studied using the ac impedance technique• Values of the average

corrosion rates over a 27-day exposure period in 3.5% NaCI solutions at pH 5.2 and pH 9.5

compared very favorably for Lockheed-prepared polyurethane-sealed and dichromate-sealed

coats of the same thickness. Average corrosion rates for both specimens over the first 7 days

of exposure compared well with those for a hard anodized, dichromate-sealed coat, but rose

well above those for the hard anodized coat over the entire 27-day period. This is attributed

both to the greater thickness of the hard anodized coat, and possibly to its inherently better

corrosion protective capability.

17-K.v Wo'ds FSoq.qeSi;_dbv-Auiho/(s))

ac impedance spectroscopy, anodized aluminum,

polyurethane sealer, dichromate sealer,

corrosion protection of aluminum

18. Distribution Statement

Unclassified - Unlimited

19 Se(:_;rity (_lassif. (of this report) .... [20. Security Classif. (of this page)

Unclassi fled ] Unclassified

21. No. of pages _2_27 Price

3 2 f NTIS

NASA FORM 1626 OCT 86

For sale by the National Technical Information Service, Springfield, VA 22161-2171

ACKNOWLEDGMENTS

The author wishes to thank Robert E. Bradley of the Lockheed Aeronautical Systems

Company for preparing the polyurethane-sealed and dichromate-sealed anodized aluminum samples.

Thanks are also due to Wendell DeWeese for photographs of the exposed sample surfaces.

TABLE OF CONTENTS

Page

INTRODUCTION .............................................................................................. I

AC IMPEDANCE DATA ANALYSIS ...................................................................... 1

EXPERIMENTAL .............................................................................................. 2

RESULTS AND DISCUSSION .............................................................................. 3

Polyurethane-Sealed Aluminum ..................................................................... 3

Dichromate-Sealed Aluminum ....................................................................... 4

CONCLUSIONS ................................................................................................ 5

RECOMMENDATIONS ...................................................................................... 5

REFERENCES .................................................................................................. 26

°o,

111

Figure

I.

2.

3.

4.

5.

,

7.

8.

9.

10.

II.

12.

13.

14.

15.

16.

17.

18.

19.

20.

21.

LIST OF ILLUSTRATIONS

Title Page

Circuit representing ac impedance response for anodized aluminum ......................... 6

AC impedance response including Warburg impedance ....................................... 7

Exploded view of the sample holder .............................................................. 8

Top view of polyurethane-sealed sample exposed to 3.5% NaCI solution at pH 5.2 ....... 9

Top view of polyurethane-sealed sample exposed to 3.5% NaCi solutions

at pH 9.5 .............................................................................................. 9

R(S)-, R(F)-, and C(F)-time curves for polyurethane seal at pH 5.2 ......................... l0

C(S)-, R(P)-, and R(T)-time curves for polyurethane seal at pH 5.2 ......................... I 1

C(DL)- and C(C)-time curves for polyurethane seal at pH 5.2 ................................ 12

/CORR and Warburg coefficient-time curves for polyurethane seal at pH 5.2 ................ 13

R(S)-, R(F)-, and C(F)-time curves for polyurethane seal at pH 9.5 ......................... 14

C(S)-, R(P)-, and R(T)-time curves for polyurethane seal at pH 9.5 ......................... 15

C(DL)- and C(C)-time curves for polyurethane seal at pH 9.5 ................................ 16

ICORR and Warburg coefficient-time curves for polyurethane seal at pH 9.5 ................ 17

Top view of dichromate-sealed sample exposed to 3.5% NaC1 solution at pH 5.2 ......... 18

Top view of dichromate-sealed sample exposed to 3.5% NaCI solution at pH 9.5 ......... 18

R(S)-, R(F)-, and C(F)-time curves for dichromate seal at pH 5.2 ........................... 19

C(S)-, R(P)-, and R(T)-time curves for dichromate seal at pH 5.2 ........................... 20

C(DL)-, C(C)-, and IcoRR-time curves for dichromate seal at pH 5.2 ....................... 21

R(S)-, R(F)-, and C(F)-time curves for dichromate seal at pH 9.5 ........................... 22

C(S)-, R(P)-, and R(T)-time curves for dichromate seal at pH 9.5 ........................... 23

C(DL)-, C(C)-, and lcokR-time curves for dichromate seal at pH 9.5 ....................... 24

iv

LIST OF TABLES

Tablc

I.

2.

Title Page

Avera,,e values of corrosion rates ............................................ : .................... 25

Composition of 606 ! -T6 aluminum ............................................................... 25

V

TECHNICAL MEMORANDUM

THE CORROSION PROTECTION OF 6061-T6 ALUMINUM BY A

POLYURETHANE-SEALED ANODIZED COAT

INTRODUCTION

This work was undertaken to investigate the corrosion prevention afforded anodized

aluminum by a newly patented invention which seals anodized aluminum in a hot water-borne

polyurethane bath [I]. This procedure is of interest in connection with corrosion problems

encountered in the water spray boiler on Ov-104, as well as with other aluminum corrosion

problems.

The electrochemical ac impedance method was the primary investigative technique

employed, with the dc polarization resistance being used where possible to corroborate values of

the corrosion currents obtained with the ac impedance method. Since the samples used in these

studies employed rather thin anodized coats, it was necessary to study dichromate-sealed specimens

with about the same coating thickness for comparative purposes.

AC IMPEDANCE DATA ANALYSIS

Two equivalent circuit models were used for analysis of the ac impedance data, and are

shown in figures I and 2. The model of figure 1 was the primary circuit model used in data analysis. The circuit model of figure 2 was used to calculate the effect of diffusion polarization. A contribution due to the Warburg impedance, or effect due to diffusion polarization, is given by

Z w _- (_to-I/2_jo-to-I/2 (l)

Here, Zw is the Warburg impedance, to = 2rr x frequency, j = X/_, and (r is the Warburg

coefficient.

The value of ¢r is obtained using the model of figure 2, and, generally, the higher the value

of tr, the less is the diffusion of the surrounding medium through the specimen coat. If the value

of tr exceeds that of the charge transfer parameter Rt, the corrosion is diffusion controlled. Trends

in or-time curves usually correlate very well with positions of maxima and minima in the ICORRtime curves, where ICORR is the corrosion current, and with the trends in R(P), the overall coating

resistance.
Values for each of the circuit components in either figure I or figure 2 were treated as

parameters in the nonlinear ORGLS [2] least squares program, which automatically adjusted these

parameters to obtain a best fit to the observed bode magnitude data (log impedance versus log to).

Corrosion currents were calculated from ac impedance data using the relation:

I('ORR = ¢h,,) × (h,) I , (2)

2.303(b,, + b, ) R, + R/

which is the Stern-Geary equation for charge-transfer control 13-5]. Tafel constants (ba and be)

were estimated to be 50 mV each, and (R,+Rf) is the total charge transfer resistance. The value of

50 mV each for the Tafel constants has been found to provide excellent agreement between values

of ICORR calculated by equation (2) and dc polarization resistance measurements [6]. The corrosion

rate for 6061-T6 aluminum is given by [6]:

Corrosion rate (mpy) = 0.42635 x ICORR (3)

EXPERIMENTAL

Polyurethane-sealed and dichromate-sealed anodized aluminum samples were supplied by the

Lockheed Aeronautical Systems Company. The samples were in the form of aluminum disks i.43

cm (0.563 in) in diameter and approximately 0.16 cm (0.063 in) thick, coated on both sides with

7.6-1.t to 12.7-1x (0.0003-in to 0.0005-in) thick sealed anodized coats. The sealing materials were

either polyurethane or dichromate. Each specimen was sanded to bare metal on one side for electrical contact, then placed in the sample holder shown in figure 3. Sample surfaces were subsequently

wiped with alcohol to remove fingerprints which would interfere with the experiments. The samples

were immersed for 27 days in 3.5% NaCI solutions buffered at pH 5.2 and pH 9.5. The buffer

solution for pH 5.2 consisted of 500 ml 0.1M potassium acid phthalate and 388 ml NaOH, with

sufficient water added to make 1 liter of solution. The buffer solution for pH 9.5 contained 500 ml

of 0.025M borax and 30 ml 0. IM HCI, with water added to make 1 liter of solution. Sufficient

NaCI was added to each solution to obtain a 3.5% NaCI concentration. A saturated calomel reference electrode was used in all measurements.

AC impedance and dc polarization resistance measurements (where possible) were made on

alternate days for the entire test period. The EG&G-PARC model 378 ac impedance system was

used for the ac impedance measurements. These data were taken in three sections. The first two

sections, beginning at 0.001 Hz and 0. I Hz, respectively, were obtained using the fast Fourier

transform technique. The last section, ranging from 5.1 to 80,000 Hz was collected using the lockin amplifier technique. The sequencing was performed automatically using the autoexecute procedure, with all data being merged to a single set for each run. The period of collection for the ac

impedance data was approximately 3 hours. After collection, the data were processed and analyzed

with an IBM PC/AT computer, which also controlled the experiment, as shown in figures I and 2.
Datal_)rthe polarizationresistancemethodwerecollectedusing the samesystemwith the

EG&G-PARCmodel 342Csoftware,which was developedespecially[or dc measurements.The

instrumentationdevelopedby EG&G-PARC,automaticallycorrectedthe dataduring the scanfor IR

drop. The datawere analyzedusingthe programPOLCURR[7]. The theory for the polarization

resistancetechniquehasbeendescribedpreviously18].

RESULTS AND DISCUSSION

Polyurethane-Sealed Aluminum

Magnified photographs of the surfaces of polyurethane-sealed anodized coats after 27 days

of exposure at pH 5.2 and pH 9.5 are shown in figures 4 and 5. The sample exposed at pH 5.2

shows general corrosion over the entire exposed surface, with one large pit appearing in the upper

right hand corner of figure 4. The sample exposed at pH 9.5 shows general corrosion over the

entire surface, with several pits evident in figure 5.

Average corrosion rates over the first 7 days of exposure and alter 27 days ot ex-posure are

listed in table 1 and compared to that [or bare 6061-T6 aluminum, identified in table 2, measured

on the first day of exposure. Results are compared to those for hard anodized, dichromate-sealed

6061-T6 aluminum, which had a much thicker anodized coat. The average corrosion rate for the

polyurethane-sealed anodized coat compared very well to that obtained with the hard anodized,

dichromate-sealed coat at pH 5.2 over the initial 7 days of exposure. Over the entire 27-day

period, however, the average corrosion rate became greater than that for the hard anodized,

dichromate-sealed sample. Curves for all parameters for the equivalent circuit models of figures 1

and 2 are shown in figures 6 through 13 for the polyurethane-sealed specimens. It may be seen

from figure 9(a) that the corrosion current rose rapidly during the last few days of exposure for the

polyurethane-sealed sample at pH 5.2, and, as a result, lead to the much higher value of the average corrosion rate observed for the 27-day period. Thus, the thickness of the anodized coat seems

to be an important factor, with the much thicker hard anodized coat giving better corrosion protection over a longer period of time. The value of the major corrosion rate controlling parameter R(T),

shown in figure 7(c), reached a minimum value after about 7 days of exposure for the

polyurethane-sealed sample, whereas the corresponding curve for the hard anodized coat reached a

minimum value after 12 days of exposure [6]. However, the Warburg coefficient-time curve in

figure 9(bt declined rather slowly when compared to that for the hard anodized, dichromate-sealed

sample [6]. which reached a minimum value after 10 days of exposure. Thus, diffusion of the

_,urrounding medium through the polyurethane-sealed coat may be a bit slower than that for the

hard anodized dichromate-sealed coat. Curves for the other model parameters for the polyurethanesealed coat related very well to those for the charge transfer resistance and the Warburg coefficienttime curves.
The averagecorrosionratesfor the polyurethane-sealed sample exposed at pH 9.5 are

shown in table I for the first 7-day period and the entire 27-day period. The average corrosion rate

for the first 7 days was greater than that for the corresponding sample at pH 5.2, but did not

increase a great deal over the entire 27-day period. This is a result of the oscillating nature of the

R(T)-time curve shown in figure I I(c), and is indicative of the healing effect previously observed

at pH 9.5 [6]. The Warburg coefficient-time curve in figure 13(b) is similar to that observed at pH

5.2. The value of the corrosion rate for the first 7-day period was greater than that observed for

hard anodized, dichromate-sealed 6061-T6 aluminum, and indicates that the corrosion protection by

the polyurethane-sealed coat was better at pH 5.2 than at pH 9.5, contrary to results obtained for

other anodized aluminum samples [6]. Values of the other parameter-time curves at pH 9.5 again

correlate very well with those lor the R(T)-time and Warburg coefficient-time curves, as for the

polyurethane-sealed sample at pH 5.2. The fact that the average corrosion rates for the

polyurethane-sealed sample at pH 9.5 were greater than those for the hard anodized, dichromatesealed sample is attributed to the relative thinness of the polyurethane-sealed coat, although a

possible increased effectiveness due to hard anodizing might be indicated.

Dichromate-Sealed Aluminum

The study of dichromate-sealed samples prepared by Lockheed provided a direct comparison

of these samples with those for hard anodized, dichromate-sealed samples. The coating thickness

for the Lockheed-prepared samples was much less than that for the hard anodized samples, and

was the same as that for the Lockheed-prepared polyurethane-sealed samples. The difference in

coating thickness must therefore be taken into account.

Photographs of the surfaces of samples exposed at pH 5.2 and pH 9.5 are shown in figures

14 and 15. General corrosion over the whole surface appears to be less at pH 5.2 than at pH 9.5.

However, large pits are evident at pH 5.2, as shown in figure 14, whereas only slight pitting

occurs at pH 9.5.

The average corrosion rate for the dichromate-sealed anodized aluminum was slightly less

than that for the hard anodized sample at pH 5.2 over the first 7-day period, but rose more rapidly

during the remaining exposure time than did that for the hard anodized specimen. Curves for

parameter-time results are shown in figures 16 through 18 for pH 5.2. Warburg-coefficient-time

curves could not be satisfactorily obtained at either pH for the dichromate-sealed samples. The

corrosion current-time curve at pH 5.2 is shown in figure 18(c) and indicated that the corrosion

rate, although oscillating, rose rather rapidly during the latter part of the exposure period.

The rate-controlling parameter R(T) reached a minimum after 10 days of exposure, which

was slightly longer than that for the polyurethane-sealed sample at pH 5.2 (7 days) and comparable

to that for the hard anodized sample [6]. Parameter-time curves for other parameters generally

followed expected trends.

The average corrosion rate for the first 7-day period at pH 9.5 was comparable to that for

the thicker hard anodized sample, but again rose to a larger value over the entire exposure period.

Parameter-time variations are shown in figures 19 through 21 for samples exposed at pH 9.5. The

R(T)-time curve in figure 20(c) indicates a minimum value after 10 days, comparable to the result
obtainedat pH 5.2. The averagecorrosionratesat both pH 5.2 and pH 9.5 arecomparableto

thosefor the polyurethane-sealedsamplesoverthe entire 27 daysexposureperiod, but are larger

thanthosefor the hard anodizeddichromate-sealedsamples.Again, this may be attributedto the

greaterthicknessof the hard anodizedsample,and, possiblyto a greatereffectivenessof hard

anodizingin corrosionprevention.The lcoRR-time curve in figure 21(c) again illustrates the healing

effect at pH 9.5. This may possibly be due to a build-up of corrosion products at pH 9.5, a

probable result ot the lesser solubility of the corrosion products.

CONCLUSIONS

The parameter-time curves for the equivalent circuit models of the ac impedance technique

generally followed expected trends. Resistance-time curves generally decrease in value with time,

while capacitance-time curves generally increased, indicating a decrease in the effective thickness

of the capacitors.

Average corrosion rates for the first 7 days of exposure for the Lockheed-prepared

specimens generally compared favorably with those for hard anodized, dichromate-sealed specimens

with a much greater thickness. The average corrosion rates over the first 7 days were compared

because of the difference in thickness, the hard anodized, dichromate-sealed coatings being 45.7

microns thick, whereas both the polyurethane and dichromate-sealed coated samples from Lockheed

were only 10.2 microns thick. However, over the extended 27-day period of exposure, the average

corrosion rates of the thinner polyurethane-sealed and dichromate-sealed samples were higher than

those for the hard anodized, dichromate-sealed samples. This effect may have been due to the

thinner coats on the Lockheed samples, but the greater effectiveness of the hard anodizing technique in corrosion protection could have been a factor.

Values of the average corrosion rates over the entire 27-day period compared very favorably

for the Lockheed polyurethane-sealed and dichromate-sealed coats. The diffusion process was

shown to have been slowed by the polyurethane seal, and the polyurethane seal was much more

effective at pH 5.2 than at pH 9.5, contrary to the results obtained for other anodized coatings.

RECOMMENDATIONS

It is suggested that the polyurethane seal be combined with the hard anodizing method and

that the coating thickness be increased, possibly providing greater corrosion protection.
Cs

IFCcII

Rs

SUBUNIT 2 .,--b

Rf

Cf

SUBUNIT 1 .--b

UNIT 2

COATING-SOLUTION UNIT

!1

Cdl

UNIT 1

METAL-COATING UNIT

Figure I.

C$

Rs

Cf

Rf

Cc

Rp

Rt

Cdl

SOLUTION CAPACITANCE

SOLUTION RESISTANCE

FARADAIC CAPACITANCE (COATING/SOLUTION)

FARADAIC RESISTANCE

COATING CAPACITANCE

COATING RESISTANCE

CHARGE TRANSFER RESISTANCE

METAL/COATING INTERFACE CAPACITANCE

Circuit representing ;,c impedance response for anodized aluminum.
Co

II

Rs Rf Rp Rt .

II

Cf Cdl

UNIT 2

COATING-SOLUTION UNIT

UNIT 1

METAL-COATING UNIT

Cs

Rs

Cf

Rf

Cc

Rp

Rt

Cdl

Zw

Figure 2.

SOLUTION CAPACITANCE

SOLUTION RESISTANCE

FARADAIC CAPACITANCE (COATING/SOLUTION)

FARADAIC RESISTANCE

COATING CAPACITANCE

COATING RESISTANCE

CHARGE TRANSFER RESISTANCE

METAL/COATING INTERFACE CAPACITANCE

WARBURG IMPEDANCE (DIFFUSION POLARIZATION)

AC impedance response including Warburg impedance.
FLAT

TEFLON

WASHER

KNURLED THUMB NUT

FLAT WASHER

WORKING ELECTRODE HOLDER

c>,...-_O-R IN G

TEST SPECIMEN

SAMPLE HOLDER BODY / HOLDER

PLUG O-RING SAMPLE HOLDER CAP

Figure 3. Exploded view of the sample holder.

8
ORrG!NAL PAGg

BLACK A[_D W_HI[E. P_JiOGRAPH

ORIGINAL FA._/"

OF POOR (_U_-Ja_(

Figure 4. Top view of polyurethane-sealed sample exposed to 3.5(// NaCI ,qolution at pH 5.2.

Figure 5. Top view of polyurethane-sealed sample expo.sed to 3.5c,4 NaCI solutions at pH 9.5.

9
POLYURETHANE SEALED

pH 5.2

(a)

1.21

_b" 0.78

X

ffl

_ 0.34

0.07 , l t L , , ,

0.30 1.69 1.92 2.15 2.30

| I I

0.53 0.76 0.99 1.23 1.46

TIME,DAYS x 10"1

(b)

3.71

N,i 2"48 1.25

O.O3

0.30 0.53 0.76 0.99 1.23 1.46 1.69 1.92 2.15 2.30

TIME, DAYSx 101

(c)

3.32

u_.2.39

.

o

:K

u.. 1.45

0.51

0.30 0.53 0.76 0.99 1.23 1.46 1.69 i .92 2.15 2.30

TIME, DAYS x 10 "I

Figure 6. R(S)-, R(F)-, and C(F)-time curves for polyurethane seal at pH 5.2.

10
POLYURETHANE SEALED

pH 5.2

(a)

4.57

IJ.

•-- 3.21

b

X

co

---- 1.86

0

0.51

0.30

JI --,.-'--'_1 I i 1 I 1 i I !

0.53 0.76 0.99 1.23 1.46

TIME, DAYS x 10-1

! I I I I I l I

1.69 1.92 2.15 2.30

(b)

2.15

1.42

=

o

,p.

X

a. 0.89

0.04

0.30

I ! i, i I ! a I

0.53 0.76 0.99 1.23 1.46 1.69 1.92 2.15

t !

2.30

TIME, DAYS x 10-1

0.94

(c)

0.63

=?,"

o

X

0.31

0.00

0.30 0,53 0.76 0.99 1.23 1.46 1.69 1.92 2.15 2.30

TIME, DAYS x 10t

Figure 7. C(S)-, R(P)-, and R(T)-time curves for polyurethane seal at pH 5,2,

11
POLYURETHANE SEALED

pH 5.2

(a)

2.02

u. 1.48

=L

.-&..J

¢3

5" o.94

0.39

0.30 0.53 0.76 0.99 1.23 1.46 1.69 1.92 2.15 2.30

TIME, DAYS x 10 -1

(b)

0.96

LI..

c. 0.67

0

o

o 0.38

0.09

0.30

I I I I I I I I I 1

0.53 0.76 0.99 1.23 1.46 1.69 1.92 2.15 2.38

TIME, DAYS x 10-1

Figure 8. C(DL)- and C(C)-time curves for polyurethane seal at pH 5.2.

J.2
POLYURETHANE SEALED

pH 5.2

(a)

6.05

O4

4.54

:=L

c_

,,'- 3.02

x

n,"

-- 1.51

0.00 I J I _ , , , l I I I I I a i I I I I

0.30 0.53 0.76 0.99 1.23 1.46 1.69 1.92 2.15 2.30

TIME, DAYS x 101

(b)

2.90

1.93 0.96

0.00 =

0.30 1.03 1.15 1.28 1.41 1.53 1.66 1.70 1.91 2.04

TIME, DAYS x 101

Figure 9. /CORR and Warburg coefficient-time curves for polyurethane seal at pH 5.2.

13
POLYURETHANE SEALED

pH 9.5

(a)

cc

3.18

2.13

1.08

0.02

0.30 0.53 0.76 0.99

I I I I I I I I I I

1.23 1.46 1.69 1.92 2.15

TIME, DAYS x 10"I

i

2.30

(b)

1.09

C= 0.70

_,-

o

_ 0.47

v

¢r

0.15

0.30 0.53 0.76 0.99 1.23 1.46 1.69 1.92 2.15

TIME, DAYS x 10"1

• I

2.38

14

(c)

1.95

U. 1.33

"1

o

0.72

0.10 _ ,

0.30

Figure I O.

0.53 0.76 0.99 1.23 1.46 1.69 1.92 2.15 2.38

TIME, DAYS x 10 "1

R(S)-, R(F)-, and C(F)-time curves for polyurethane seal at pH 9.5.
POLYURETHANE SEALED

pH 9.5

(a)

iJ.

C

O3

0

3.81

2.73

1.65

0.56

0.30 0.53 0.76 0.99 1.23 1.46 1.69 1.92 2.15 2.30

TIME, DAYS x 10"1

(b)

4.81

3.24

"T"

1.66

0.08

0.30

'-.._'_i i _ '

0.53 0.76 0.99 1.23 1.46 1.69 1.92

TIME, DAYS x 10 "1

l I I 1

2.15 2.30

(c)

1.56

_ 1.14

%-

X

0.71

CC

0.28

0.30

I

0.53 0.76 0.99 1.23 1.46 1.69 1.92 2.15 2.30

TIME, DAYS x 10-1

Figure I1. C(S)-, R(P)-, and R(T)-time curves for polyurethane seal at pH 9.5.

15
POLYURETHANE SEALED

pH 9.5

(a)

0.61

0.42

x

_" 0.24

0.06

0.30 0.53 0.76 0.99 1.23 1.46 1.69 1.92 2.15 2.30

TIME, DAYS x 101

(b)

2.91

14.

" 1.97

O

_" 1.03 0

0.09 I I . i l I I I I I i I i I l I I i I

0.30 0.53 0.76 0.99 1.23 1.46 1.69 1.92 2.15 2.38

TIME, DAYS x 101

Figure 12. C(DL)- and C(C)-time curves for polyurethane seal at pH 9.5.

16
POLYURETHANE SEALED

pH 9.5

(a)

2.67

E

:=L 1.93

d

q,...

X

n,"

_ 1.20

0.46

0.30

! I J J I I J I I J ,I ! I J I I i ,L I

0.53 0.76 0.99 1.23 1.46 1.69 1.92 2.15 2.30

TIME, DAYS x lO 1

(b)

3.09

"7,

_ 2.06

"T"

0

"- 1.03

X

0.00

0.30 0.51 0.72 0.93 1.14 1.35 1.56 1.77 1.90 2.19

TIME, DAYS x 10 "1

Figure 13. /CORR and Warburg coefficient-time curves for polyurethane seal at pH 9.5.

17
ORIGINAL PAGE

BI.kCK AND W_HJT.F.,PHO]_OGRAP_t.-i

ORIGINAL PAGE rS

OF POOR QUALITy

Figure 14. Top view of dichromate-sealed sample exposed lo 3.5_,_ NaCI solution at pH 5.2.

Figure 15. Top view of dichromate-sealed sample exposed to 3.5% NaCI solution at pH 9.5.

18
DICHROMATE SEALED

pH 5.2

(a)

0.54

b 0.35

,rX

GO

_" 0.16

0.03

0.30 O.53 0.76 0.99

i _ I I I I I I I I I

1.23 1.46 1.69 1.92 2.15 2.38

TIME, DAYS x 10-1

(b)

1.45

v 0.96

(,,3 =

T,-

X

0.48

E:

0.01.

O.3O 0.53 0.76 0.99

I *_, --* I I I I I I I I I

1.23 1.46 1.69 1.92 2.15 2.38

TIME, DAYS x 10 1

(c)

0.57

0.39

LL

-t

_ o.22

0.05

O.3O 0.53 0.76 0.99 1.23 1.46 1.69 1.92 2.15 2.38

TIME, DAYS x 10 "1

Figure 16. R(S)-, R(F)-, and C(F)-time curves for dichromate seal at pH 5.2.

19
DICHROMATE SEALED

pH52

(a)

1.32

1.00

1,1..

C

_," 0.68

O

X

Or)

O"" 0.36

0.03 , .--_.--._r---.-,.,,__

0.30 0.53 0.76

[

0.99 1.23 1.46 1.69 1.92 2.15 2.38

TIME, DAYS x 10-1

(b)

0.87

v 0.58

0

!-.

X

_" 0.29

nr0.00 I I I I I I I I I. 1 1

0.30 0.53 0.76 0.99 1.23 1.46 1.69 1.92 2.15 2.38

TIME, DAYS x 10-1

(c) O

2.09

1.35

X

0.60

0.53 0.76 0.99

0.14 , ,,,,-'1"-- , j i I I I i I A i

0.30 1.23 1.46 1.69 1.92 2.15 2.38

TIME, DAYS x 10-1

20

Figure 17. C(S)-. R(P)-. and R(T)-time curves lor dichromate seal at pH 5.2.
D.ICHROMATE SEALED

pH 5.2

(a)

4.75

•"_ 3.15

.,J

0

1.56

0.04

0.30 0.53 0.76 0.99 1.23 1.46

TIME, DAYS x 10-1

I I I I I ! l

1.69 1.92 2.15 2.38

(b)

1.27

0.87

_-

0

X

0.48

o

O

0.09

0.30

1 !

0.53

_""''1"'_ | I ! | I I I I I | I ! 1 I

0.76 0.99 1.23 1.46 1.69 1.92 2.15 2.38

TIME, DAYS x 10 -1

(c)

3.20

E

_ 2.12

-1

T,-

x 1.04

rr

0

0

0.04

0.30

! I I

0.53 0.76 0.99 1.23 1.46 1.69 1.92 2.15 2.38

TIME, DAYS x 10 1

Figure 18. C(DL)-, C(C)-, and /CoRR-time curves for dichromate seal at pH 5.2.

21
DICHROMATE SEALED

pH 9.5

(a)

1.85

1.59

c= 1.32

v

_ 1.06

x 0.79

O_

r,- 0.53

o.26

0.00

0.30 0.53 0.76

I I I I I I I | I I I I

0.99 1.23 1.46 1.69 1.92 2.15 2.38

TIME, DAYS x 10 -1

(b)

3.70

_2.48

9

X

_- 1.26

0.03

0.30 0.53 0.76 0.99 1.23 1.46 1.69 1.92 2.15 2.38

TIME, DAYS x lO -1

(c)

2.08

1.81

1.54

u. 1.27

:=L

1.00

x 0.72

_0.45

0.18

0.30

_q n

0.53 0.76 0.99 1.23 1.46 1.69 1.92 2.15 2.38

TIME, DAYS x 10"1

Figure 19. R(S)-, R(F)-, and C(F)-time curves for dichromate seal at pH 9.5.

22
DICHROMATE SEALED

pH 9.5

(a)

4.75

kk.

C

_:_ 3.46

X

2.18

O

0.90

0.30 0.53 0.76

I I | I I I I I I I I ! I I

0.99 1.23 1.46 1.69 1.92 2.15 2.38

TIME, DAYS x 10-1

(b)

2.70

v 1.81

¢,_1=

O

T--

X

_" 0.91

0.02

0.30 0.53 0.76 0.99

i ,,.,I,. I I I ! ! I I I I I

1.23 1.46 1.69 1.92 2.15 2.38

TIME, DAYS x 10 -1

3.36

'v"

"t" 2.23 c_

T--

(c) ,,

n.. 0.72

0.05

0.30 0.53 0.76 0.99 1.23 1.46 1.69 1.92 2.15 2.38

TIME, DAYS x 10-1

Figure 2(). C(S)-. R(P)-. and R(T)-timc curves i_r dichr_malc seal at pH 9..5.

23
DICHROMATE SEALED

pH 9.5

(a)

2.57

LL

:=L 1.84

.-J

Q

o 1.10

0.35

O.3O 0.53 0.76 0.99 1.23 1.46 1.69

TIME, DAYS x 10-1

1.92 2.15 2.38

(b)

4.10

2.78

,OJ"

b

l,-

X

A 1.47

o

O

0.15

0.30

I I

0.53 0.76 0.99 1.23 1.46 1.69 1.92 2.15 2.38

TIME, DAYS x 10-1

24

(c)

1O48

_' E 1.03

:=L

0.58

X

Oo0.13

D

0.09

0.30 0.53 0.76 0.99 1.23 1.46 1.69 1.92 2.15 2.38

TIME, DAYS x 10 1

Figure 21. C(DL)-, C(C)-, and /CORR-time curves for dichromate seal at pH 9.5.
Table I. Average values of corrosion rates

Material Thickness

606 I-T6 AI Sea.__._l Microns pH

Average Corrosion

Rate, First 7 Days,

Mils/year

Bare Aluminum* -- -- 9.5 0.75

5.2 1.91

Hard Anodized CR207 - 45.7 9.5

Aluminum 5.2

0.00113

O.00020

Anodized Polyurethane 10.2 9.5

Aluminmn 5.2

0.0319

0.00026

Anodized Cr207-- I0.2 9.5

Aluminum 5.2

0.00031

0.00003

*Corrosion rate measured on first day only.

Average Corrosion

Rate, 28 Days,

Mils/year

0.0032

0.0058

0.0545

0.0398

0.0196

0.0577

Table 2. Composition of 6061-T6 aluminum

Listed Weight Percent

Element (The Aluminum Association, Inc.)

Cu 0.15-0.40

Mg 0.8-1.2

Ti O. 15 max

Fe 0.7 max

Si 0.4-0.8

Mn 0.15 max

Zn 0.25 max

Ni 0.05 max

Cr 0.04-0.35

AI Balance

Observed Weight Percent

(MSFC Analysis)

0.36

1.01

0.01

0.50

0.62

0.04

0.13

0.02

0.17

Balance

25
REFERENCES

I. Bradley, R.E., and Keithler, W.R.: Industrial Finishing. Vol. 5, 1988, p. 30.

2. Busing, W.R., and Levy, H.A.: "A General Non-Linear Least Squares Program." ORGLS,

Oak Ridge National Laboratory, 1958.

3. Stern, M., and Geary, A.L.: Journal of the Electrochemical Society, Vol. 102, 1955, p. 609.

4. Stern, M., and Geary, A.L.: Journal of the Electrochemical Society, Vol. 104, 1957, p. 56.

5. Stern, M.: Corrosion, Vol. 14, 1958, p. 440t.

6. Dantord, M.D.: "The Corrosion Protection of Aluminum by Various Anodizing Treatments."

NASA Technical Memorandum 100366, April 1989.

7. Gerchakov, S.M., Udey, L.R., and Mansfield, F.: "An Improved Method for Analysis of

Polarization Resistance Data." Corrosion, Vol. 37, 1981, p. 696.

8. Danford, M.D., and Higgins, R.H.: "An Electrochemical Study of the Corrosion of Primer

Coated 2219-T87 Aluminum." NASA Technical Paper 2459, April 1985.

26
APPROVAL

THE CORROSION PROTECTION OF 6061-T6 ALUMINUM

BY A POLYURETHANE-SEALED ANODIZED COAT

By Merlin D. Danford

The information in this report has been reviewed for technical content. Review of any information concerning Department

of Defense or nuclear energy activities or programs has been

made by the MSFC Security Classification Officer. This report,

in its entirety, has been determined to be unclassified.

J iWf Montano

Corrosion Research Branch

Paul M. Munafo /_ "

Chief

Metallic Materials Division

@aul H. Schuerer

Director

Materials & Processes Laboratory

_'U.S. GOVERNMENT PRINTING OFFICE 1990-731-061t20082